The present invention relates generally to the field of motor vehicles and, more particularly, is directed to an improved motor-driven power assist system for steering vehicles.
Power steering systems for motor vehicles are well-known in the prior art. Such systems generate an auxiliary steering force in accordance with steering by the driver and the force thus generated is transmitted to a steering gear which turns the wheels. Most of the power steering systems in use at the present time are actuated by a hydraulic drive. The hydraulic drive is equipped with a control valve, hydraulic cylinder and related structure which generates an auxiliary steering force by moving hydraulic fluid within the cylinder in accordance with steering by the driver.
A significant disadvantage of hydraulic power steering systems, especially in light of the trend toward smaller cars, is that the control valve, hydraulic cylinder and related structure are large in size. The hydraulic lines must also be formed with a small curvature to prevent pressure loss in the system. In the case of vehicles not having a large mounting space in the engine compartment, such as front wheel drive vehicles, these constraints make it difficult to mount hydraulic power steering systems. Hydraulic systems must also be effectively sealed to prevent fluid leakage. Thus, the maintenance and installation of hydraulic power steering systems is quite troublesome.
In order to avoid the above-identified problems, it has been proposed to use an electric motor as the driving means for power steering systems. While motor-driven steering systems represent a significant improvement over hydraulic systems, electric motors are prone to a high failure rate at elevated temperatures. These failures have chiefly to do with break-down of the electrical insulation in the motor due to heat, causing the motor to burn out.
Because power steering assist devices must often be installed in the vehicle's engine compartment where relatively high temperatures are expected, the problem of motor failure in motor-driven steering systems is a significant one. The problem is aggravated when the motor is in continuous use as the electric current flowing through the motor causes it to heat up even further. Thus, there is a need in the art for a motor-driven power steering system which protects the motor from premature failure at elevated temperature.